The use of quicklime and similar alkaline materials for sanitation has a long history. The Roman army was known to spread quicklime in their latrines. In the last century, with the development of modern sewage systems, the use of lime products for flocculation of solids in raw sewage or liquid sludges has been developed. The role of alkaline materials in pH adjustment and the beneficial effect of pH in pathogen reduction is a more recent development, having occurred in only the last 60 years. The end result of research into pathogen reduction in wastewater treatment has prompted the U.S. Environmental Protection Agency to promulgate regulations specifying environmentally sound treatment criteria. More specifically, the EPA's "Process Design Manual for Sludge Treatment and Disposal" outlines methods for achieving pathogen reduction through pH adjustment using lime or other alkaline materials.
The EPA regulations outline two levels of pathogen reduction; the first being a Process to Significantly Reduce Pathogens (PSRP treatment), and a second higher level of pathogen inactivation or Process to Further Reduce Pathogens (PFRP treatment). Disposal or agricultural reuse of sludges processed under a PSRP treatment are still highly regulated because unwanted pathogenic activity can still occur in the end-product. From a marketing standpoint PFRP treatment is more desirable. A PFRP end-product is virtually pathogen free and unsuitable for regrowth of undesirable organisms, consequently the regulatory focus is on the process itself, not multiple end-use land application sites.
In the EPA's design manual the lime or alkaline processes described achieve a PSRP level of stabilization, through only pH adjustment. Not all pathogens can be inactivated through pH adjustment. Parasite ova, such as hookworm or Ascaris are known to survive for long periods of time in high pH conditions, consequently an alternative method of inactivation must be used to kill parasites. The EPA design manual states in part that with such a method:
Direct addition of dry quicklime to sludge and without the use of a separate slaker, is practiced in Denmark [and]in at least ten Swedish treatment plants. Potential advantages are the elimination of slaking equipment and the generation of heat, which can improve pathogen reduction and speed dewatering through evaporation. In one case, direct additions of dry quicklime were made to raise sludge pH above 13.0 and bring the temperature to 175.degree. F (80.degree. C). Salmonella and intestinal parasites were killed within two hours. Heat generated by slaking of quicklime does not raise temperature significantly unless the sludge is dewatered and the lime dose is high--on the order of 400 to 800 pounds per ton dry solids (200-400 kg/t).
The recent history of advanced alkaline stabilization processes to produce a PFRP end-product stem from these early European efforts.
A majority of research in this field involves the pathogenic stabilization of sludge through pH adjustment and heat generated from the exothermic reaction of quicklime with water. U.S. Pat. No. 4,270,279 issued to Roediger and U.S. Pat. No. 4,306,978 issued to Wurtz disclose methods utilizing such research. U.S. Pat. No. 4,270,279 discloses the gentle handling of partially dewatered sludge cake and dusting only the surfaces of sludge particles thus resulting in a granular product; however, this process can only be carried out using partially dewatered sludge cake in sheet form prepared by belt filter presses.
U.S. Pat. No. 4,306,978 discloses a process which utilizes dewatered sludge cake from any source, however a high dose of quicklime is necessary in carrying out the process. The thorough mixing of the quicklime in this process, as compared to only surface dusting in the Roediger process, is a major reason for the increased quicklime usage in this process. In actual tests using the same type sludge blender as described in U.S. Pat. No. 4,306,978, that being the blender described in U.S. Pat. No. 3,941,357, it was found that 25 percent more quicklime was required to produce a granular product than is needed to meet EPA PFRP pH and temperature standards.
Granularization or pelletization of sludges has been taught in a number of disclosures such as U.S. Pat. No. 3,963,471 issued to Hampton, U.S. Pat. No. 4,956,926 issued to Glorioso, and U.S. Pat. No. 5,069,801 issued to Girovich. Each of these patents teach the recycling of a portion of the pelletized end-product to promote further drying and pelletization. The drawback to each of these systems is that all of the recycle material must be reheated. A finished product cannot be drawn off directly after the recycle mixing.
Accordingly, there is clearly a need for a process and apparatus which can meet the ERA PFRP ph and temperature standards while minimizing the amount of quicklime necessary in carrying out the process. Moreover, there is a need for a process and apparatus which efficiently and effectively achieves a virtually pathogen free end-product which is unsuitable for the regrowth of undesirable organisms. Furthermore, there is a need to minimize dust and pasteous end product and to produce a durable granular or pelletized sludge product for use as a soil amendment or fertilizer through a more energy efficient granulation or pelletization method.